Light sources having a high luminous flux and a high luminance are employed in a wide variety of fields, for instance in endoscopy and likewise in projection apparatuses, wherein gas discharge lamps are currently the most widely used for this purpose. In lighting applications, for example projection or endoscopy, on the basis of LARP (“Laser Activated Remote Phosphor”) technology, which is known in principle, a phosphor is excited by a laser arranged at a distance from said phosphor. In this case, the laser radiation that impinges on the phosphor is at least partly converted into wavelength-converted useful light by means of wavelength conversion by the phosphor.
The term laser radiation hereinafter encompasses both non-visible, e.g. ultraviolet (UV) or infrared (IR), laser radiation and visible, e.g. blue-violet, blue, red, etc., laser radiation. A suitable phosphor or a phosphor mixture converts the invisible or visible laser radiation into corresponding visible electromagnetic radiation, i.e. light. Hereinafter the term “color” phosphor, where “color” here is representative of a concrete color, for example one of the primary colors red, green, blue, yellow, etc., or a mixed color composed of two or more primary colors, characterizes a phosphor which, upon excitation with suitable laser radiation, converts the latter into light having the relevant “color”, i.e. what is meant here is a light color, rather than a body color. A red phosphor thus converts suitable laser radiation, for example blue laser radiation of a blue laser diode having an emission wavelength of approximately 460 nm, into light having the light color “red” (red light), a green phosphor converts the laser radiation into light having the light color “green” (green light), etc. The invention is not restricted to the visible range for the superimposed light, although this is preferred from a present-day perspective for practically relevant applications. For video projecting, in particular, the corresponding phosphors for the projector color channels red, green and blue (possibly also further color channels, e.g. yellow) are usually applied to a rotating wheel in order to distribute the laser power over a larger area on average over time and thus to reduce the phosphor degradation. In addition, static phosphor arrangements are also known, in which the phosphors are applied on a heat sink. At all events the light wavelength-converted by a phosphor is collected by means of an optical device, e.g. reflector, converging lens or TIR optical element (TIR: Total Internal Reflection; e.g. conical glass rod), and used further for the relevant application.
What is disadvantageous is that red phosphors have a lower conversion efficiency in comparison with yellow and green phosphors if they are irradiated with laser radiation having high surface power densities (e.g. 10-50 W/mm2). As a result, for red light, in particular, limits are imposed on the luminous fluxes and luminances that can be achieved with LARP technology.